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Series pumps and my hydronic comedy of errors

I've searched and read posts here about using pumps in series to obtain additional head to overcome loops that are too long in in-floor hydronic systems. I think I know what I’m going to do, but wanted to ask for comments first. As an incentive to help and for your collective amusement, I will first provide a list of the comedy of errors associated with this system.

Background – this is 1,615 square foot home consisting of two, uninsulated buildings about 15 feet apart. Construction is stuccoed concrete block with high (14 foot) ceilings that comprise a concrete roof topped with Spanish tile, There are many large wood-framed single pane windows and doors throughout both buildings. It was built in 2009, the owner died about six months later, and we purchased the home in 2011.

The home is 7,200 feet above sea level, in Pátzcuaro, México, where in-house heating systems consist of a gas or wood fireplace or portable ventless gas heater at best, or more typically, nothing at all. Insulation is never used in construction, and gaps around doors and windows are standard. Low temperatures in December and January are in the low 40s with daytime highs in the low 60s. Summer rainy season can be cold and damp after a few days without sunshine. People wear a lot of wool sweaters and bundle up in quilts at night during both seasons, not your typical image of México. We cheat at night with electric mattress pads, and run unvented portable gas heaters and a gas-log fireplace when it’s cold.

Unvented gas appliances pump a huge amount of water vapor into the air, which then condenses and runs down on the cold window glass, staining the woodwork. (Weather station data and webcam that we installed last November can be seen at http://tinyurl.com/pfbm7b7 by entering a custom date range beginning on Nov. 5, 2013.)

The home has an improperly installed in-floor radiant heat system. It was the architect’s first and only system, and he relied on an installer who claimed to know what he was doing. He lied. Here’s what I’ve discovered:

The Comedy of Errors:

1. The installer imported a boiler, mixing valve assembly, and zone controls from Spain and Italy that are 230 volt 50 cycle. Mexico is 127 volt, 60 cycle. They fixed the voltage problem with a step-up transformer, but the 50 Hz pumps are running 20% faster on 60 Hz A.C. In our case, this is not a bad thing.

2. They insulated under the slab but not around its edges. The concrete floors extend under the outside walls and doors to various outdoor patios.

3. The construction photos and architects’ drawings were lost when his hard disk crashed. The paper plans were lost when the original owner died and friends emptied the house. There are no engineering designs for the system – no heat loss or sizing

calculations. No one knows the tubing layout or lengths.

4. The 15-foot underground PEX runs from the manifolds to the main building were uninsulated. I’ve since dug them up and insulated them.

6. Thermostats were placed on outside walls instead of centrally located in their rooms. Also fixed by me.

7. Mixing assembly bypass valves were closed, making what should have been a primary/secondary loop system into a single loop.

8. The differential valve was at its lowest setting, so it was always bleeding.

9. The balancers were misadjusted.

10. The 1” copper pipes in the boiler room were all uninsulated.

11. The big one – they ran 1/2" PEX in the slab with only three zones, all longer than 300

feet.

Analysis:

I’ve added pressure gauges, a flow meter, a SpiraVent, and made as many corrective adjustments as possible. I’m running a 176F boiler with the mixing valve set to its maximum of 131F. Supply and return manifold delta-T is 40F, and delta-P is 11.75 psi with one zone on, 11.0 psi with all three zones on. Flow with all three zones on is 2.45 gpm with all balancers wide open. Flow for the longest zone by itself is 0.67 gpm.

Using tables of PEX pressure drop, I estimate my loop lengths at 829, 432 and 576 feet. Zone areas are approximately 715, 412, and 488 square feet, excluding the connected areas outside the perimeter walls. I characterize these zones with Cv’s of 0.202, 0.289 and 0.247 respectively.

Zone 1 is the living room, dining room and kitchen in an open floor plan. Only the dining room floor and part of a hallway gets warm. There isn’t enough head to push warm water fast enough to the rest of the loop.

Zone 2 is the master bedroom, bath and walk in closet. It gets warm enough to be comfortable, though the floor temperature is far from uniform.

Zone 3 is the guest bedroom, bath and studio in the second building. The guest bedroom comes up to temperature, but the studio receives no heat and requires a portable gas heater to be comfortable on cold evenings.

The secondary loop pump is a Wilo RS25/7, developing 7 meters of head at 50 cycles, or 8.4 meters at 60 cycles. That’s 27.56 feet or 12 psi, which corresponds to my measurements. I’ve posted a PDF with system drawings and photos at

Page 3 is a graph of my estimated system curves as well as pump curves of the present pump and pumps I’m considering to put in series.

Page 4 is a photo of the 18 KW boiler (less derating for altitude) and mixing value assembly.

Page 5 is a photo of the supply and return manifolds.

Page 6 is a photo showing a typical floor section in the main house.

I also have graphs of the boiler supply and return temperatures as well as the manifold supply and return temperatures, recorded with a 4-probe Hobo data recorder that I had left over from an old engineering project. But I think that’s overkill for this discussion – I’ve adjusted things so the boiler no-longer short cycles when supplying two zones.

My thought at this point is to add a 230 volt, 60 cycle Grundfos UP26-116F or UPS26-150FC downstream of the secondary Wilo circulator. The graph in the PDF shows how I expect the system curve to look with additive heads. The three-speed UPS25-150FC can be dialed back to a lower speed if there’s a problem with such a large head mismatch with the Wilo. However, my reading on this forum indicates that such problems are most likely at the right-hand, low head, flow-cutoff end of the pump curve, not at the high-head, low-flow end of the curve where this system will operate.

I would like to have some confirmation that the heads will be additive and that there won’t be problems of cavitation or wind milling.

For those that have made it this far in this overly long post, I would appreciate any thoughts and comments. I should also mention that finding these pumps in Mexico is difficult, and they typically cost 2.5 times what they do in the US, so trying and swapping if it doesn’t work isn’t much of an option. I’d like to go with a best guess, and have someone driving here from the US bring the pump and flanges with them. Swapping out the existing Wilo is not an option because it’s European and uses 1.5” threaded unions on 180 mm centers, whereas all the US pumps seem to be flange based. That’s why I’d like to use two pumps in series and develop a whopping 32 psi (73.7 feet) of head to see if I can finally push the water through fast enough to spread the heat around the floor. If flow increases as the square root of pressure, going from 11 to 29 psi (all zones on) should increase my flow rate by 62 percent (square root of 29/11).

A secondary consideration is the high cost of propane and electricity here. The present system will eventually make the main house and the guest bedroom comfortable, but it runs very hard and consumes a lot of gas. I’d rather spend more on electricity and have the slab be more uniform in temperature.

Thanks in advance for anyone’s thoughts, including those who might say I should just abandon the system and buy more sweaters.

If I went to the lumber yard to buy a nice piece of cherry wood to make a nice table top and it had a big bow in the board, I have to cut or plane the "Hag" out if the board to get it usable. Once I get the board straight, the finished board will be narrower. No matter what I do, the board will be narrower. I can either make a smaller top or I can buy another board that won't match the first board.

Installing radiant tube under a floor is like the cherry wood table top. They don't sell wood width or length stretchers in the specialty tool stores. You won't find pipe stretchers for under floor use at the Internet tool stores either. You're stuck with what you have. Sort of like breeding two really dumb dogs of unknown ancestry. You probably won't get a smart dog.

No matter what you try to do to this system, it isn't going to work to your expectations. Its time to abandon this fools errand. Everyone before you was a fool and you need to stop running errands for them. Go find some Mesoamerica ruins and try to dig up a broken pot. Try to find all the pot shards. If you're lucky enough to find all the pieces, try putting them together and making a whole pot. That doesn't leak. Either way, no matter what, you still have only a broken pot. Repaired, but it's still a broken pot.

@kcopp - I considered through the wall heaters by Rinnai. They no longer sell them in Mexico, and because of many doors and windows, there are few locations in the outside walls with suitable spacing for the heaters. It would also require a lot of cutting in the masonry to get gas and power to these locations, plus the addition of ceiling fans to bring the heat down from the high ceilings.

@icesailor - I said it was a comedy. Thanks for your constructive suggestions.

@zman - Reversing the flow to the manifolds was my very first thought on this project, but was advised that seated zone valves were unsuited, and my valve seats are integral with the return manifold. Here's a cross section of the valve and actuator: https://dl.dropboxusercontent.com/u/12631920/Caleffi_Valve_Seat_and_Actuator.pdf . I just called Caleffi technical support, and learned that these valves are fine with reverse flow, and with up to 125 psi of differential pressure should I go with boost pump.

@Chris - Excellent suggestion -- thanks. I can do it by weight or volume.

Look guys, I know it's a ridiculous system, but we're generally in a milder climate than what you're used to working with. Besides, it's what I've got to work with. During the coldest months, I used $100 of propane in 21 days, and it kept most of the house pretty darn comfortable, if not uniformly or efficiently heated. Friends would come in from their cold houses and be thankful they could finally remove their sweaters. I would have been happy with heating bills like that during the 40 years I lived in the mountains of Colorado.

Back to my original post, since the logistics of getting stuff here requires a fair amount of planning, anyone see a problem with putting the Grundfos high-head UPS26-150FC in series with the existing Wilo? If I can get heat through half of the longest loop, then the 4-way mixer becomes a good way to finish the job.

It is actually just pressure. These pumps are tested with a certain condition, and one critical condition is the inlet pressure to the pump. If it is less than specified, the manufacturer can't guarantee performance meeting the performance curves they generate.

"Available" means that you can't take a 1/4" line, connect it to the pump, with required static pressure, then turn on the pump and expect it to perform. The inlet pressure will drop due to the pipe friction.

Got to give credit to John Siegenthaler, who introduced me to the alternate meaning of NPSH. Kind of like A.S.M.E., Always Sometimes, Maybe EXCEPT, or A Substantial Monetary Exchange…. Either applies :-)

ME

It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.

It's actually even worse than you say, though, Mark -- if you don't have enough NPSHA, not only will the pump not meet its performance curve, it may cavitate so badly as to destroy itself, sometimes in minutes.

I've seen it...

Jamie

Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

I just read Grundfos' white paper on NPSH(A) and did a back of the envelope calculation for a second pump. Could you walk through this with me?

I run my system at 1.2 bars, so H(a) is 40.7 feet cold (observed 1.6 - 1.7 bars hot). H(s) is the delta-P of the first pump at my new projected flow rate with two pumps, about 24 feet. H(vpa) at 60C is 6.7 feet. I'll give 2 psi over to friction losses, or 4.6 feet. So my ballpark NPSH(A) at the inlet to a second pump would be 40.7 + 24 - 6.7 - 4.6, or 53.4 feet. I can't find NPSH(R) in any Grundfos literature, but elsewhere I've seen minimum inlet pressures on the order of 1 psi or smaller for similar circulators, so I think I would be a very long way from cavitation if I understand the concept and calculation correctly.

If anything, I presently have more of a cavitation danger at the inlet to the existing Wilo pump, because H(s) is effectively zero.

Elsewhere I've read that series pumps get into trouble at the low-head end of the curve, where the first pump is producing only a few feet of head, thereby lowering NPSH(A) at the second pump.

And if cavitation were happening, it seems I could raise NPSH(A) by bumping up the system rest pressure [H(a)] from 1.2 bars to 1.5 or more. The boiler spec says it can go to 3 bars when operating, though I wouldn't want to be up that high. Increasing the system pressure in this way directly increases Vapor Pressure and keeps bubbles from forming.

Another guy here in town who used to do commercial plumbing in Reno, keeps telling me that systems run better with higher static pressure. Since I was seeing the world only in terms of delta-P's, I couldn't fathom what he was talking about. But now I think I might understand -- liquids are more likely to stay as liquids and not produce bubbles with higher system pressures.

Thanks to all for getting me to think deeper about how things actually work.

Mark, I would be careful about series pumping with circs of drastically different performances (I would always recommend the circs be the same size in this application). Reason being, if the big circ is downstream of the smaller one it can potentially suck the system fluid out of the smaller circ. If the big circ is upstream it will make the downstream small circ ineffective, with it's energy overpowering the small circ.

Regarding NPSHr, all wet rotor circs require a "minimum inlet pressure", not NPSHr as with a dry rotor circ. This is to stop the water from boiling (flashing into steam) in the rotor/can area, killing the sleeve bearings.

Typically small circs are approx 2 PSIG min met pressure with the pump running. Larger ones can go as high as 14 PSIG.

I kind of think reverse flow might the best solution. Or how about some electric glass as a supplemental heat source???

Don't forget to buy a base for your timer and be sure to read the manual carefully. Some of these timers start the sequential timing as soon as power is applied. Some are powered all the time and need a start signal to initiate and a reset signal to stop. Make sure you get the correct coil voltage.

This is seriously F-U. You are trying to put a band aid on a tumor and hoping it will go away.

Putting another pump in series with another is just another way of installing a multi-stage pump. Have you ever taken apart a 2 stage water pump and seen one with two different sized impellors? NO. Submersible pumps are designed to PUSH water from deep depths while submerged in water. The higher the pressure it needs to PUSH the water up, the more stages of impellors the pump needs. ALL Impellors are the same size. In the case of what you are trying to do, the ONLY way it MIGHT possibly work is if the first pump is a very high volume but low pressure/head developing pump. It must deliver a far greater amount of water to the second pump than it can ever use. If not, the marbles will be rolling in the case and the metal termites will be eating the impellors.

I don't quite understand the issue of reversing the flow. In MY opinion, the pump/s should be pumping INTO the restriction. If you are considering pumping away from the restriction, you increase the likelihood of cavitation.

Look at it this way. With a big enough pump, you can push water to the moon. If you tried to do the same from the moon and sucked it, you can only LIFT water 25' to 27' practically. You can lift water 33' theoretically. Also, think of it like draft in a chimney. You can only increase the suction pressure to a pure vacuum. But you can blow up the same chimney with compressed air. You can pump air to the moon with a big enough compressor and a long enough pipe.

The other thing that many fail to consider is that the cavitation comes around the edges of the impellor/propeller. Look at the wing vortex's around jet aircraft. Its because of negative air pressures at the tips and allowing the temperature and dew points to form clouds. Sonarmen on Submarines listen to the cavitation signature sounds of ships. Submarines make the same sounds. The deeper they go, the greater the pressure, and the less (if any) cavitation sounds. Look at pictures of boats passing overhead from below, underwater and see the stream of bubbles coming off the tips of the prop blades. Stand at the back of a boat/ship and look at the wake. All white and full of bubbles. From prop cavitation and friction cavitation from any surface that the water passes through.

If you try anything, you need to run the system as high as you can (25#) and increase the expansion availability. Like more or bigger bladder tanks.

But understand that in the world of pumps, very unusual things go on when you mismatch pumps.

Honeywell, Johnson Controls and others offer actuators. I went with Belimo as they had great tech support and directed me to the best actuator for the application. They suggested the 90 in/lbs. knowing that hydronic valves tend to get sticky in poor water conditions.

Google actuators, or damper actuators and find all sorts of suppliers.

I imagine most brands offer mounting kits to adapt to the stem dimension and bracket to the valve.

You can also find other brands of 4 way valves, I like the tekmar quality, I doubt they forge the valve themselves, probably find it with other names, Danfoss, ESBE, etc.

I have salvaged a few 1000 foot single loop radiant jobs installed by DIYers with the reverser trick. All were pleased to get the slab to a comfortable level with this method.

This troubles me a little, as I have done it with mismatched pumps and not one of the many pitfalls mentioned in this thread cropped up. I installed gauges to monitor performance and the pumps worked exactly as I planned. The 2 pumps were a 0011 and a 2400-50. I calculated the total system head @ gpm and played with that variable to find exactly where to plot on the pump curves. In my feeble mind, as long as both pumps are able to generate a net positive pressure on the outlet @ the gpm the system will perform at, there is no problem.

One thing that becomes extremely important though with this scenario though, You have to have a minimal psi drop (minimal pipe distance) between the inlet of the first pump and the expansion tank. Pumping away of course.

Correcto. And you understand what the concept is. If you have sense and knowledge enough to use gauges, you won't get into trouble.

Put another way, some of us understand that it is no problem trying to fill a 10# bag, that already has 5# of cow chips in it, with another 3# of chips. Some others don't understand that if you have a 5" bag and it is full of chips, you can't put another 3# of chips in a full bag.

And the spacing between pumps or ells from pumps is important for the fluid to get their sheets back together.

Thanks for the detailed information and photos. I'm going with the Tekmar 721 1" mixer valve for my crossover. The Belimo spring-return NFB24-X1 (24 VAC) or NFBUP-X1 (Universal Power 24-240 VAC) you pointed me to seems to offer better performance in terms of operating times than Tekmar's own 741 actuator (53 in-lb, 105 second running time) at a fraction of Tekmar's price. Belimo NF's are 90 in-lb with operating times of less than 75 seconds by motor and 20 seconds by spring return. Since my manifolds will be partially bypassed while switching direction, it seems the shorter run time will be more efficient if I'm reversing every 30 minutes. Plus, the Belimo can be controlled with an SPST contact, whereas the Tekmar requires an SPDT contact. The NFBUP can be run on 120 VAC and saves the cost of a 24 VAC transformer.

I just spoke with Jeff at Belimo tech support. You're right about them being very helpful. It seems I can do even better with their non-spring return NMX series, which are even less expensive than the spring return NF series. Jeff said that unless there's a need for fail-safe operation, which there isn't in my case, go with motor driven in both directions. Further, he said I can get 150, 95, 60 or 45 second operating time in both directions by specifying the desired time when placing the order. http://www.valve-actuator-warehouse.com has every conceivable Belimo listed on their website. The NMX120-3 runs on 120 VAC and will operate with an SPST contact closure.

When I described my situation -- hydronic, too-long loops, desire to reverse flow with the Tekmar 721, Jeff said I was looking at exactly the right actuator with the NMX series.

@Icesailor -- regarding your comment "I don't quite understand the issue of reversing the flow. In MY opinion, the pump/s should be pumping INTO the restriction. If you are considering pumping away from the restriction, you increase the likelihood of cavitation."

Flow reversal just ahead of the manifolds doesn't change whether I'm pumping into or away from the restriction of overly-long loops. I would always be pumping into the restriction, but just from opposite directions.

John Siegenthaler's excellent 2004 article on flow reversal, "The Long Way Around", seems to be impossible to get to in the archives of Plumbing and Mechanical magazine. However, I found it cached by Google, and created this short URL that will get you to the cached version if you're interested in reading it.

Nope -- even though it's power open and power close, no double-throw relay is needed. Have a look at the first of the two wiring diagrams on page 2 of the specs to see how to run it off a single contact closure:

Here's the most inexpensive timer I could find -- about ten bucks -- that should do the job with a Belimo NMX120-3, 100 to 240 VAC non-spring return actuator that only requires an on/off signal -- no 24 volt transformer or SPDT relay needed. It's mechanical and can be programmed with 96 integral 15-minute on/off pegs. Seems perfect for achieving flow reversals that are multiples of 15 minutes.

Google "Apollo 6 Timer" and lots of on-line retailers have them. I prefer mechanical to digital because my experience is that digital timers sometimes lose their programming due to surges or power spikes.

The hydroponic grow industry seems to be awash in timers, which is how I first found the Apollo 6. It's made by Titan Controls.

I'll have to run a separate, always-on 120 VAC line to two of the terminals of the Belimo, and then another wire coming from the timer outlet to the Belimo's 120 volt control input.

I think I've now reached a decision on all the components needed to achieve flow reversal. I now have to decide on a secondary boost pump -- either match the existing Wilo or risk going with a larger pump with 3-speed capability that I can dial back if there are problems.

It been my experience that timers are not a good place to skimp. They have some very intricate mechanisms inside that make them work.

If you do end up have to add series pumping on top of doing the reversing, couple things I would do.

Make sure you are pumping away from the expansion tank.

In order to plot your system curve, add a pressure gauge to the inlet and outlet of the current pump. If the pump does not currently have isolating flanges on, put some on. Also install a flow meter, Caleffi makes some really nice ones. You can now make measurements. You need to record 3 separate points of gpm vs pressure drop on the gauges. The first point with the pump on and both isolator flanges open. The second point, slowly start closing one of the isolator flanges till you see the gpm drop and so on.

1. GPM @ PSI ΔP

2. GPM @ PSI ΔP

3. GPM @ PSI ΔP

You can repeat this process for each zone. With the 2 data points from each zone, the hydraulic resistance can be calculated. Then you can use the following formula to map the system, simulating different flow rates and seeing what the head loss would be accordingly.

Head loss = (hydraulic resistance) x (flow rate)^1.75

I think this is a much better method than poking around in the dark. It will help you select the proper pumps. Also, depending on the situation, you may want to wire the auxiliary pump to only come on when a specific zone or combination thereof, signals a call for heat.

The reviews of this ten buck timer from the hypdroponic people on Amazon were all very positive. At this price, I can buy two and keep one as a spare, or use it on my lamps when we go away.

I have spent a great deal of time attempting to characterize the system, installing pressure gauges and a Caleffi 0-5 gpm flow meter. One problem is that the flows on the longest zone at 11.75 psi are less than 1 gpm and not easy to read accurately on the Caleffi.

To better characterize the system, I put a 25 psi regulator on a garden hose and timed the flow on each zone into a 5 gallon jug. Granted, it's a 60F not 125F water, but it gave me a good estimate of Cv.

All of this information (pump curves, system curves), along with photos and system diagram are in the PDF file that was referenced in my first post if you have time to look at them. The included graph there shows estimated head if I combine the existing Wilo with either of two possible Grundfos circulators.

As for distance between pumps, a booster pump would be about 5 feet downstream from the existing Wilo on 1" copper, with a ball valve, two 90 elbows, and a Spiravent air eliminator in between.

My advice, scratch Series Pumping completely. There is a point where series pumping is no longer a good option, and your system is well beyond that point. It would take an incredible amount of electricity to generate the pressure needed and that pressure would have to be maintained for a very long time to satisfy zone 1. You would have to install a differential bypass for those pumps, as you simply don't want to be running large pumps with so little flow.

All is not lost though. If you can get your hands on a thermal imaging camera, you should be able to map the tubing in zone 1. Run hot water through one way, map it. Then switch the loops on the manifold and run it through the other way to map the other half. You just might find a convenient location about halfway through the loop to cut a small hole in the floor, split the loop and make 2 runs back to the manifold.

If you get that accomplished and do the 4-way reversing, you should be in high cotton.

I would go with the reversing valve. Your loops would still be too long based on industry standards even if you were able to split them. It's kind of like when Alaska became a state. The governor of Texas wanted Alaska to be split in half because he wanted Texas to remain the biggest state, The Alaskan representative said "Okay, then we'll have two states bigger than Texas".

Another thing to worry about is the venting (or lack there of) on that boiler. It looks like it's just pointed out of the window. Take care of that before you worry about which way to pump. You won't be able to enjoy the comfort if you're not around to feel it.

I understand. Pumping IN TO the restriction is the only way you can make it work. I didn't understand your concept of being able to pump IN TO the return to make it flow backwards. Sounds like a serious exercise of "Fun With Fittings". Another hydronic piping Picasso in the works. Go for it.

I appreciate your suggestions, but my chances of finding a thermal imaging camera in Michoacan are about the same as my winning the lottery. I’ve done a little tracing with a hand-held infrared thermometer, but once the heat runs out in the tubing, there’s nothing to trace. And even if I was lucky enough to find a point to split the zone near an outside wall, I’d have to channel through 30 feet of polished concrete and three steps in the surrounding patio to get the new supply and return lines to the manifold. It’s just not feasible.

The existing Wilo seems to have held up fine in the low-flow, 27-foot head regime with a differential valve. I don’t see why a second, similarly sized pump wouldn’t also hold up, especially since both would now experience additional flow. I’ll have to relocate the existing differential valve or install a new one downstream of the second pump.

Flow reversal seems to be the most straight-forward first step, even if it results in what icesailor calls a “hydronic piping Picasso”.

Good catch on the informal stack venting. I did not include it in my original “comedy of errors” list because it wasn’t directly related to hydronics. But when I first saw it, it was another “What the Mexico?!?!?” moment (a friend’s expression). This is an externally vented flash heater with a co-axial stack, something the architect apparently wasn’t knowledgeable about. It came with a co-axial elbow, but they ran it into a simple horizontal tube through the wall. When it wouldn’t operate properly (exhaust gas cycling right back into the fresh-air intake), they rotated the elbow 90 degrees and cut the rectangular hole in the wall that you see in the photo. It fires just fine now.

The boiler is in a small bodega (storage room) with an outside door just out of the picture. It’s not a living space, and we only enter to get things out of storage. There’s no monoxide danger with the wide-open door if the gases don’t make it out the window, which in fact they seem to do. At some point, I’ll make a proper co-axial tube and reconnect the elbow as it was supposed to be originally and close up the opening.

I’ve got someone flying down on June 13th, so I just ordered the parts I need to put the flow reversal idea into practice. I'll report back on the results.